High energy solid propellants for use in rocket motors having ratios of nitro glycerin or other nitrate ester plasticizer to binder as high as 2.5 present significant storage problems in that the nitrate esters tend to migrate to the surface of the propellant. Such nitrate esters tend to be absorbed in flame inhibitor coatings and rocket casing liners and insulators, degrading their performance.
Flame inhibitor coatings are necessary in rocket motor applications to control the burning surface area of the grain, thus providing a steady supply of propulsive gas at constant pressure for as long as several minutes. Flame inhibitor coatings are applied to the grain surface of rubber based propellants where burning is not desired to accomplish this result. No prior inhibitors are known that are useful for propellants having nitrate ester plasticizers. Polymeric liner and inhibitor materials such as polyesters, polyethers, and polybutadienes with other compounds tend to absorb the nitrate esters migrating from high energy propellants, rendering the coating flammable and subject to peeling from the grain surface. Propellant grains having such a degraded coating are subject to uneven burning resulting in rocket malfunction.
Rocket casing liners and insulators also tend to absorb nitrate esters from the solid propellant resulting in softening of the liner. Liners and insulators are in part relied upon to inhibit burning of the propellant along the walls, and absorption of the nitrate esters renders them flammable resulting in possible rocket motor failure. Present practice includes the physical imbedment of nitrocellulose particles in the liner along the interior surface of the liner or insulator before casting of the propellant therein to inhibit nitrate ester migration, a process which is complex and must be carefully controlled.
The coating of a gun propellant with a siloxane polymer to function in part as a flame retardant to reduce peak pressure within a gun barrel when fired using the propellant is taught in U.S. Pat. No. 4,263,069 to Reed, Jr. et al. The use of this relatively non-flexible coating on a solid rocket propellant, however, would be undesirable. Rocket casings can be manufactured of filament wound reinforced polymeric materials which are subject to physical expansion under the high pressures of firing, resulting in dimensional changes in the solid propellant grain. Any flame retardant coating must, thus, be flexible so as to continue adhering to the grain surface without cracking, peeling, or breaking away during rocket motor firing. The siloxane coating of the prior art is not flexible, nor is it required to be so since a gun propellant is not subject to such dimensional changes as encountered in rocket propellants. Thus, the siloxane coatings of the prior art would not be useable for rocket propellant flame retardant coatings.